I've installed win7 x64 on my late 2012 MBA 13" and when i boot in windows there is a black screen with a line flashing for about 1 minute and after that starts loading windows. Is this delay ok? Or i have to do something?

I thought this had to do with the boot loader. If you are using some kind of windows activation remover it'd explain a lot. If you are using a legitimate windows copy, I remember there were ways to modify the boot loader to work better with Apple computers, might want to look into that.